Mechanics of cryopreserved aortic and pulmonary homografts

The surgical placement of pulmonary valve grafts into the aortic position (the Ross procedure) has been performed for three decades. Cryopreserved pulmonary valves have had mixed clinical results, however. The objectives of this study were to compare the mechanics of cryopreserved human aortic and pulmonary valve cusps and roots to determine if the pulmonary root can withstand the greater pressures of the aortic position.

Six aortic and six pulmonary valve roots were obtained from the Oxford Valve Bank. They were harvested during cardiac transplantation from hearts explanted for dilated cardiomyopathy (mean patient age 68 years). The whole roots were initially stored frozen at -186 degrees C, then shipped packed on dry ice. After complete thawing, the roots were pressurized whole; test strips were then cut from the valve cusps, roots and sinuses and tested for stress/strain, stress relaxation, and ultimate failure strength.

The pulmonary roots were more distensible (30% versus 20% strain to lock-up) and less compliant when loaded to aortic pressures. The pulmonary valve cusp and root tissue also showed greater extensibility and greater stiffness (lower compliance) when subjected to the same loads.

We conclude that mechanical differences between aortic and pulmonary valve tissues are minimal. The pulmonary root should withstand the forces imposed on it when placed in the aortic position. However, if implanted whole, the pulmonary root will distend about 30% more than the aortic root when subjected to aortic pressures. These geometric changes may affect valve function in the long term and should be appreciated when implanting a pulmonary valve graft.